W. L. Menninger

Multimegawatt relativistic harmonic gyrotron traveling-wave tube amplifier experiments

The first multimegawatt (4 MW, /spl eta/=8%) harmonic (/spl omega/=s/spl Omega//sub c/, s=2,3) relativistic gyrotron traveling-wave tube (gyro-twt) amplifier experiment has been designed, built, and tested. Results from this experimental setup, including the first ever reported third-harmonic gyro-twt results, are presented. Operation frequency is 17.1 GHz. Detailed phase measurements are also presented. The electron beam source is SNOMAD-II, a solid-state nonlinear magnetic accelerator driver with nominal parameters of 400 kV and 350 A. The flat-top pulsewidth is 30 ns. The electron beam is focused using a Pierce geometry and then imparted with transverse momentum using a bifilar helical wiggler magnet. The imparted beam pitch is a /spl alpha//spl equiv//spl beta//sub /spl perp////spl beta//sub /spl par///spl ap/1. Experimental operation involving both a second-harmonic interaction with the TE/sub 21/ mode and a third-harmonic interaction with the TE/sub 31/ mode, both at 17 GHz, has been characterized. The third-harmonic interaction resulted in 4-MW output power and 50-dB single-pass gain, with an efficiency of up to /spl sim/8% (for 115-A beam current). The best measured phase stability of the TE/sub 31/ amplified pulse was /spl plusmn/10/spl deg/ over a 9-ns period. The phase stability was limited because the maximum RF power was attained when operating far from wiggler resonance. The second harmonic, TE/sub 21/ had a peak amplified power of 2 MW corresponding to 40 dB single-pass gain and 4% efficiency. The second-harmonic interaction showed stronger superradiant emission than the third-harmonic interaction. Characterizations of the second- and third-harmonic gyro-twt experiments presented here include measurement of far-field radiation patterns, gain and phase versus interaction length, phase stability, and output power versus input power.

Long‐pulse millimeter‐wave free‐electron laser and cyclotron autoresonance maser experiments

Experimental results on high‐power long‐pulse free‐electron laser (FEL) and cyclotron autoresonance maser (CARM) experiments are summarized. Single‐mode operation of a free‐electron laser oscillator at 27.4 GHz with a Bragg resonator has been obtained, with an output power of 990 kW for a beam energy of 320 keV and transmitted current of 30 A, corresponding to an efficiency of 10.3%. Free‐electron maser (FEM) amplifier operation at 35 GHz has yielded a gain of 26 dB with an output power of 800 kW, corresponding to an efficiency of 8.6%. CARM oscillator experiments at 32 GHz with a different electron gun have yielded lower powers because of poor beam quality; planned CARM experiments are discussed.

Cyclotron autoresonance maser (CARM) amplifiers for RF accelerator drivers

Cyclotron autoresonance maser (CARM) amplifiers are under investigation as a possible source of high-power (>100 MW), high-frequency (>10 GHz) microwaves for powering the next generation of linear colliders. The authors have completed the design study for a high-power, short pulse, 17.136 GHz CARM amplifier, utilizing a 500 kV linear induction accelerator. A three period bifilar helical wiggler with a wiggle wavelength of 9.21 cm and a field of up to 50 G will be used to spin-up the electron beam. For CARM designs based on induction accelerators, the peak beam powers and beam pulse durations result in the generation of RF pulses which are already well suited to the requirements of future colliders. In addition, a unique feature of CARM amplifiers that use a bifilar helical wiggler to spin-up the electron beam is that the wiggler can be designed so that the phase stability of the CARM is substantially enhanced. Results of the design study, as well as final design parameters, are presented.<<ETX>>

Autophase cyclotron autoresonance maser amplifiers

The phase stability of the output power from high‐power microwave tubes is a critical issue in applications such as electron acceleration and radar. The radio frequency (rf) phase stability of cyclotron autoresonance maser (CARM) amplifiers is investigated, and the phase stability of a CARM amplifier with a suitable correlation between perpendicular momentum and beam energy is shown to result in an order‐of‐magnitude improvement in phase stability over other rf sources. A technique for producing such a correlation, termed autophasing, is identified.

CARM and harmonic gyro-amplifier experiments at 17 GHz

Cyclotron resonance maser amplifiers are possible sources for applications such as electron cyclotron resonance heating of fusion plasmas and driving high-gradient rf linear accelerators. For accelerator drivers, amplifiers or phase locked-oscillators are required. A 17 GHz cyclotron autoresonance maser (CARM) amplifier experiment and a 17 GHz third harmonic gyro-amplifier experiment are presently underway at the MIT plasma Fusion Center. Using the SRL/MIT SNOMAD II induction accelerator to provide a 380 kV, 180 A, 30 ns flat-top electron beam, the gyro-amplifier experiment has produced 5 MW of rf power with over 50 dB of gain at 17 GHz. The gyro-amplifier operates in the TE/sub 31/ mode using a third harmonic interaction. Because of its high power output, the gyroamplifier will be used as the rf source for a photocathode rf electron gun experiment also taking place at MIT. Preliminary gyro-amplifier results are presented, including measurement of rf power, gain versus interaction length, and the far-field pattern. A CARM experiment designed to operate in the TE/sub 11/ mode is also discussed.<<ETX>>

CARM amplifier theory and simulation

Abstract The theory and simulation of cyclotron autoresonance maser (CARM) amplifiers are presented, including studies of amplifier phase stability, multimode phenomena, and susceptibility to absolute instabilities. Recent results include particle-in-cell simulations of the onset of absolute instabilities and numerical modeling of multimode effects. Phase stability studies indicate that the output phase of CARM amplifiers may be relatively insensitive to fluctuations in beam energy, pitch, and current, for optimized designs; simulations show a phase sensitivity of ∼ 2° per percent beam energy variation. An experimental design for a long-pulse 17 GHz CARM amplifier is presented.